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The Biology and Developmentof Brachygaster minutus Olivier(Hymenoptera: Evaniidae), a parasiteof the oothecae of Ectobius spp.(Dictyoptera: Blattidae).Valerie K. Brown aa Department of Zoology , Royal Holloway College (LondonUniversity) , Englefield Green, Surrey, U.K.Published online: 17 Feb 2007.

To cite this article: Valerie K. Brown (1973) The Biology and Development of Brachygaster minutusOlivier (Hymenoptera: Evaniidae), a parasite of the oothecae of Ectobius spp. (Dictyoptera:Blattidae)., Journal of Natural History, 7:6, 665-674, DOI: 10.1080/00222937300770561

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J. nat. Hist., 1973, 7 : 665-674

The Biology and D e v e l o p m e n t of Brachygaster minutus Olivier (Hymenoptera: Evaniidae), a parasite of the oo thecae of Ectobius spp. (Dietyoptera: Blattidae).

VALERIE K. BROWN

Department of Zoology, Royal Holloway College (London University), Englefield Green, Surrey, U.K.

Introduction The numerous parasites, commensals and symbionts of cockroaches have

been reviewed by Roth and Willis (1960). The morphology and taxonomy of the British species of a small group of parasitic Hymenoptera, the Evanioidea, have been studied fully by Crosskey (1951), although in this work repeated reference is made to the extreme paucity of information on the habits and life histories of this group. Crosskey recognizes three families and in one of these, the Evaniidae, there are only two known British species; Evania appendigaster (L.) for which no precise localities are cited, and Brachygaster minutus (Olivier) which has a widespread distribution in Europe, though British records are rare. Townes (1949) suggests that all species in this family are parasites of cockroach ootheeae. Remarkably little is known of the biology and development of B. minutus although several related species have been studied (Cameron, 1957; Edmunds, 1952 a, b, 1954, and Cros, 1942). The present paper gives some account of these aspects in B. minutus which has been found to parasitize the oothecae of the three British species of field-dwelling cockroaches belonging to the genus Ectobius. The work falls into five sections, viz.: (i) adult biology, (ii) mating, (iii) oviposition behaviour, (iv) development, and (v) adult emergence.

Material and methods Collection o] specimens

B. minutus can either be collected as an immature stage within the oothecae of Ectobius, or as a free-living adult. Adults were successfully collected only from sand-dune localities, e.g. Studland Bay, Dorset, where the interior of large tufts of marram grass, Ammomophila arenaria (L.) Link can be carefully examined. However, it frequently took several hours to locate a single specimen. At the other sites oothecae were collected either from bracken debris or from grass tussocks and a proportion of these subsequently proved to be parasitized. The type of habitat selected for ootheca deposition makes this latter method even less practical.

During this study specimens of B. minutus were collected from the following areas in small numbers:

Host cockroach

species Locality

E. lapponicus North Gravel, Imperial College Field Station, Silwood Park, Berks.

Crown Es ta te Plantat ions, Nr. Braeknell, ]~erks.

Nat ional grid

reference

SU947688

SU881664 SU884662

J.N.H. 2 W

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666 Valerie K. Brown

Grassland, Zoology Department, Royal Holloway College, Surrey SU 996698

E. pallidus Hengistbury Head, Nr. Christchurch, Hants SZ 175908 E. panzeri Canforcl Heath, Nr. Wimborne Minster, Dorset SZ 024955 E. pallidus/ Studland Bay, Isle of Purbeck, Dorset SZ 037850

E. panzeri

I t would seem likely tha t B. minutus is commonly present in localities which support large populations of a species of Ectobius, and tha t the paucity of records for the parasite may be due to the considerable difficulties experienced in its collection.

Culture Single parasitized oothecae were kept for their entire developmental period

on a layer of moist sand in 5.0 × 1.0 cm glass tubes, closed by a plug of finely teased cotton wool. Adult parasites were reared in round, plastic cages (10 cm in diameter and 4 cm high), with a layer of fine sand covering the base. A moistened cotton wool roll provided a supply of water, and several moistened sultanas were given as food. Folded filter paper strips afforded resting sites and shelter for the adults. Oviposition by the parasites was easily induced in the laboratory. Single oothecae were offered to mated females at regular intervals and removed after oviposition. This technique resulted in a higher proportion of oothecae being parasitized than the alternative method in which many oothecae were introduced into the cage at one time and left for the female's entire life. The latter method encouraged repeated oviposition in a small number of oothecae. Parasitized oothecae containing late larval instars can easily be distinguished from unparasitized oothecae when viewed with transmitted light. The outline of the larva or pupa can be seen as it becomes slightly withdrawn from the periphery of the oothecae unlike the developing cockroach embryos.

An additional difference is apparent later in development since prior to hatching unparasitized oothecae of Ectobius increase in size and become pale in colour due to water uptake, whereas parasitized oothecae do not alter their shape or colour at this time.

Results Adult biology

B. minutus has been found to emerge from field collected oothecae of all three British species of Ectobius. I t is solitary in habit with only one adult emerging from an ootheca. Since the host is killed during the parasite's development, this species should perhaps be termed a parasitoid or even a predator as several host eggs are devoured (Doutt, 1959). The parasites emerge during the summer months when adult cockroaches are abundant (table). The adults of both sexes are agile creatures which become increasingly active in the afternoon and early evening, when mating and oviposition commonly occur. Their usual method of locomotion is to run swiftly over the substrate, though occasionally they climb nearby vegetation. The males fly sporadically for short distances. Both sexes move with extreme rapidity when pursued and it is this which makes their collection so difficult; however, they become far less active towards the end of their adult life.

Mating and oviposition may occur soon after emergence. In an outdoor insectary the mean longevity (quoted with standard error) of adult males was

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Biology and development of Brachygaster minutus Olivier 667

l~elationship between the t ime of emergence of the parasi te and the adul t cockroaches, for one season

Host Emergence period Emergenee period of species of B. minutus adult cockroaches

E. lapponicus 22n4 J u n e - 1 0 t h August 24th ~¢[ay-10th August E. pallidus 7th Ju ly -29 th Ju ly 30th J u n e - 2 6 t h August E. panzeri 5th August-23rd August 20th Ju ly -6 th September

21-2 + 2-4 days (n= 22) and adult females 38-9 +_ 2.7 days (n= 25). However, the longevity of both sexes is reduced to a few days if food is not provided. The feeding habits of this species in the field are not known; although related species have been reported to feed on certain flowers (Edmunds, 1954).

Mating behaviour The adults of the two sexes can be readily distinguished by the colour of the

fore and mid tibiae which are black or dark brown in the male but yellow in the female (Crosskey, 1951). Both sexes arc capable of mating immediately after emergence from the ootheca, and may mate more than once. The beginning of courtship behaviour is marked by a general increase in activity of both sexes followed by frequent cleaning of the antennae. In this both antennae are simultaneously drawn downwards with the forelegs and then passed through the month parts. The male, with wings raised, often rubs the side of his abdomen with the hind legs. The fore and mid, and mid and hind pairs of legs may also be rubbed together. The male then actively pursues the female for about 30 sec or until she eventually stops (usually on the lid of the cage) and turns to face him. Antennal fencing begins and finally the male runs behind the female and mounts her with his wings raised. The female responds by raising her abdomen dorsally with the genitalia extruded and the exposed male genitalia grip those of the female. Copulation lasts for 13-20 see (n = 30) during which the male continuously strokes the antennae of the female. The male frequently loses contact with the lid of the cage and hangs suspended from the female. At the completion of mating the male regains his hold on the lid and walks away. After mating the female may become aggressive towards the male. I f several males are confined in a cage with a single female the males fight and may become damaged.

Oviposition behaviour Females of B. minutus oviposit readily in oothecae of the three species of

Ectobius without displaying any obvious preference. The species of ootheca from which the parasite emerged does not seem to affect any subsequent choice of host. Mated or virgin females oviposit immediately after emergence in oothecae which have been deposited for various lengths of time, although recently deposited ones are definitely preferred. However, the parasites show no interest in oothecae which are still being carried by a female cockroach. Oothecae which have previously been buried by a cockroach after deposition are frequently uncovered by the parasite. The parasite does not necessarily feed before oviposition. A very precise behavioural sequence accompanies oviposition.The female parasite approaches the ootheea with rapidly vibrating antennae (fig. 1 (a)). This is followed by a thorough exploration of the ootheca using the antennae and is continued for several minutes, the female often

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Biology and development of Brachygaster minutus Olivier 669

walking away and returning to the ootheca. The parasite then climbs on to the ootheca, with antennae still drumming (fig. 1 (b)), and walks over it several times in an at tempt to balance it (fig. (1 (c)). Finally the female assumes a position parallel to the longitudinal axis of the ootheca, the head is bent forwards and the antennae are flexed over the end of the ootheca and moved slowly. The mid and hind legs are used to anchor the female to the ootheea and the fore legs are held over the end of the ootheca. The abdomen is then lowered and the ovipositor extruded and drills through the ootheca wall. The female typically remains in this position for periods varying from 22-60 min (n = 23) (fig. 1 (d)) and then withdraws her ovipositor and walks away (fig. 1 (e)). At no time was the female observed to feed from the oviposition puncture. During the oviposition period the oothecae frequently move slightly; however, oviposition continues without interruption, the female often lying on her side at its completion. I t is worth noting tha t the time taken for oviposition increases considerably at the end of the season, one female taking 5 hours for a single oviposition. This may well be correlated with the increasing hardness of the ootheca. The female usually returns to the ootheca and attempts to bury it in the sand (fig. 1 (/)). The fore and mid legs are used to scrape the sand over the ootheca, whilst the hind legs remain on the ootheca to steady it. The female continually changes her position so that all sides of the ootheca are covered. The sand is finally built up into a dome 2-3 cm in diameter with the ootheca in the centre; this may take up to 15 min.

The orientation of the ootheca before oviposition seems to be important. A female will make definite attempts to roll an ootheca, which has been placed with the keel upwards, on to its side. On one occasion a female parasite carried an ootheea between its legs for several centimeters before depositing it on its side and commencing oviposition. The head may be used to reorientate an ootheca prior to oviposition, or alternatively a female may run repeatedly over an ootheca and rock it into a more suitable position. I f several females are enclosed in a cage in which a female is engaged in oviposition the other females will at tempt to disturb her and often move the ootheca with their heads.

A single egg is laid within a cockroach egg at each oviposition. One female is capable of parasitizing many ootheeae, though the exact fecundity is not known. However, it is certain tha t this is much lower than in many other hymenopterous parasites of oothecae where a large number of eggs are laid at a single oviposition (e.g. Tetrastiehus hagenowii (Roth & Willis, 1954); Anastatus sp. (Ananthasubramanian & Ananthakrishran, 1961), although the longevity of Brachygaster is greater. Very occasionally two females oviposit into different eggs in the same ootheca, but since only one larva develops these extra eggs are presumably consumed along with the cockroach eggs.

Development Parasitized oothecae were fixed (using Alcoholic Bouins Fixative) at regular

intervals from oviposition until the emergence of the adult the following summer. This revealed that the hatching of the egg and the development of the early larval stages proceed rapidly, since at the end of one month under outdoor conditions a mature last instar larva has been formed. The parasite overwinters in this stage and does not undergo any further morphological change until a prepupa is formed in May or June.

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0,1 mm

l m m r i

(a) (b)

l f om : (d)

Fzo. 2. Developmental stages of Brachygaster minutus. (a) Egg. (b) Third instar larva. (c) Prepupa. (d) Pupa (shading indicates pigmentation).

In order to locate the parasite the fixed oothecae were dissected. The egg is laid within a cockroach egg and is orientated to lie parallel to the longitudinal axis of the latter. Only a single egg is laid at each oviposition, measuring 0.49 mm in length and 0.14 mm in width. The egg is creamy white in colour with a smooth chorion. I t is spindle shaped and slightly curved in outline.

The number of larval instars was established by examining the mandibles (which vary in shape in each instar) remaining in the ootheca at the end of larval development. A total of five larval stages could be distinguished. All the instars are typical of parasitic Hymenoptera with 13 body segments of which the last three are generally smaller and retracted. All the larval instars are white in colour. The head capsule is spherical and the cephalic skeleton which is prominent supports a pair of distinctive mandibles above the labium and maxillae. At the end of each instar the larva becomes spherical and loses all trace of segmentation.

The first larval instar remains within the cockroach egg in which it was laid. The mandibles are triangular in shape, sharply pointed but only moderately sclerotized. The second and third instars have mandibles of a similar shape although the degree of sclerotization is greater in the third instar. They are essentially triangular in shape and have two sets of fine teeth behind the main denticle. The second and third instar larvae start consuming other host eggs and at the end of the third instar almost half the eggs have been consumed. I t is likely that the well developed rows of fine teeth assist in tearing the chorions of the cockroach eggs.

In the fourth and fifth instars the mandibles become very strongly sclerotized and bidentate with a larger ventral denticle and smaller dorsal one;

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Biology and development of Brachygaster minutus Olivier 671

however, the rows of teeth are lost. The mandibles become much blunter and heavier in the last instar and the cephalic skeleton more pronounced. The mature fourth instar occupies about three-quarters of the ootheeal volume and only the eggs at one end are undamaged. The last instar generally consumes the remainder of the contents of the ootheca such that no trace of the eggs remain with the exception of a pile of torn chorions. The larva assumes a very wrinkled appearance and the head and terminal body segments are retracted. The larva lies with its longitudinal axis parallel to the keel and its dorsal surface pressed against it. I t is at this stage that overwintering occurs.

There is a distinct prepupal stage which is passed through quickly. This is again white in colour and closely resembles the mature larva except for well marked constrictions between the future head, thorax and abdomen of the imago. The exarate pupa is at first white in colour but gradually darkens, the abdomen, antennae and distal leg segments are the last structures to pigment. Finally the whole pupa becomes dark and shiny. Edmunds (1954) has recorded three distinct stages in the development of the pupa of Prosevania punctata (Brulld) each based on the degree of pigmentation.

(a) (~)

0 - 0 5 mrn I I

FIG. 3,

(c) 0 ' 0 5 mm

1,, I

O . 0 5 m m

Brachygaster minutus: (a)-(e), mandibles of successive larval instars (shading indicates degree of sclerotization).

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Emergence Movements of the adult can be seen within the ootheca just before

emergence. The adult parasite emerges from the ootheca by making a hole in the wall of the ootheca. Firstly the wall is punctured by the mandibles and a small quanti ty of moisture is released, the small hole is gradually enlarged by the parasite biting pieces from the margins of the aperture. I t seems likely that the secretion may contain enzymes which soften the wall to facilitate this. The head and thorax of the parasite can be seen as the hole becomes larger and frequent at tempts are made to escape through the hole by the entire body being pushed forwards. The enlargement of the hole is continued until the fore-legs emerge followed by the rest of the body. On emergence the body of the parasite is dry, deeply pigmented and the wings are fully expanded. The emergence hole is always situated at one end of the ootheca in a ventro-lateral position under the keel. The interior of an ootheca from which a parasite has emerged is devoid of the internal lattice structure formed by the membranes of the individual cockroach eggs. The only remnants are the cast larval and pupal skins and a layer of grey material in the ventral part of the ootheca, thought by Edmunds (1954) to be an accumulation of excretory material from the larval stages. Occasionally one or two cockroach embryos are not consumed by the parasite, but these always fail to hatch, presumably because ' ha t ch ing ' is dependent on the combined action of all the embryos (Brown, 1973 b.

The time of emergence of the parasite from the oothecae of the three species varies and is completely synchronized with the occurrence of the adult cock- roaches of the relevant species in the field. The first parasites emerge after the early host adults have been in the field for some time and have commenced deposition of oothecae (table). The mechanism involved in this synchroniza- tion is not clear, but the results are consistent. I f parasites collected at the end of the season are allowed to oviposit in oothecae of the three species, which are then kept in an outdoor insectary, the emergence of the adults the following summer will invariably fall within the range for that species given in the table.

From a total of 24 parasites which emerged from field-collected oothecae in one season the sex ratio was unity. However, in the case of unfertilized females all the progeny are males (arrhenotoky). The adult parasites vary in size, though generally males tend to be larger than females. However, the main cause of the size difference appears to be the species of ootheca from which the parasite emerged. Oothecae of E. panzeri produce smaller parasites than the other two species; the largest adults emerge from oothecae of E. lapponicus. This may be explained when the size of the ootheeae of the three species (Brown, 1973 b, is considered, since the difference in size will govern the amount of available food material. The possible effect of the species of ootheca on the sex ratio of the offspring is at present under study.

Discussion Cameron (1957) suggests that all Evaniids breed more rapidly than their

hosts. This is apparently true for those species which parasitize the domestic cockroaches; P. punctata, a parasite of the oothecae of Blatta orientalis L. and Periplaneta americana (L.), has three generations a year (Edmunds, 1952 a) whilst either two or three annual generations have been recorded by Cros (1942)

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Biology and development of Brachygaster minutus Olivier 673

from the oothecae of the former species. E. appendigaster, another parasite of the oothecae of P. americana, has three or sometimes even four generations a year. However, Edmunds (1954) records that Hyptia Illiger species, which parasitize the oothecae of the native wood roaches (Parcoblatta Hebard spp.) in North America, like B. minutus, have only a single generation a year. The explanation of this difference may be in the fact that in household species of cockroaches there is a constant supply of adults and thus the synchronization of the life cycle of the parasite with that of the host brought about by a reduction in the number of generations a year, is not necessary.

The quiescent stage in the life cycle of B. minutus is the last larval instar. The two Hyptia species which parasitize the oothecae of wood roaches also overwinter at this stage (Edmunds, 1954). The ootheeae of Ectobius are known to be in a state of diapause during the winter months (Brown, 1973 a), and generally diapause in the host is accompanied by a cor- responding arrest in the growth of the parasite (e.g. Andrewartha & Birch, 1954). This ensures the synchronization of the life cycles of the parasite and its host. However, the exact mode of overwintering in this parasite is not yet fully understood and requires much further work, since this may well serve to clarify the complete and interesting synchronization in the time of emergence of host and parasite which occurs when the species parasitizes different species of Ectobius. This feature is a most valuable adaptation in the life cycle of the parasite, since most habitats support populations of only one host species.

The importance of the parasite in the ecology of the cockroach species is difficult to assess. From the limited data available it seems unlikely that more than 10% of the egg capsules are parasitized. The only comparable estimate of parasitization by Evaniidae involves the two species of Hyptia mentioned previously which parasitized 6.7% of the overwintering oothecae of wood roaches (Edmunds, 1952 b), although this varied annually. The higher levels of parasitism, for example, those cited by Cameron (1957) for E. appendigaster attacking ootheeae of P. americana, are probably confined to the household species of cockroaches which support several generations of parasites annually.

Summary Brachygaster minutus is a parasite of the oothecae of three species of British

field-dwelling cockroaches belonging to the genus Ectobius. This species has previously received very little attention probably due to its restricted occurrence in Britain and the difficulties encountered in its collection. The life cycle and development of the parasite is described. The species is solitary in habit, the single larva overwintering within the ootheea as a final instar. The emergence of the adult the follo~fug summer is perfectly synchronized with the occurrence in the field of the particular cockroach species. Aspects of the mating and oviposition behaviour of the parasite are described.

Acknowledgments This work was commenced at Imperial College Field Station under the

tenure of a S.I%.C. Studentship. I wish to thank Mr. G. E. J. Nixon for confirming the identification of the parasite and Dr. M. J. Copland for critically reading the manuscript.

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R e f e r e n c e s ANATIIUSUBRAMANIAN, K. S. & ANATtIAKRISHNAN, T . W . 1961. The biology of Anastatus sp.

(Eupelmid~e: Hymenoptera) parasite on the oothecae ofSupella 8upellectilium. J. Zool. Soc. India, 13 " 62-69.

ANDREWARTHA, H. G. & :BIRCH, L. C. 1954. The Distribution and Abundance of Animale. Chicago: University Press, 782 pp.

BROWSr, V. I'1:. 1973 a. The overwintering stages of Ectobius lapponicus (L.) (Dietyoptera: Blattidae). J. Ent. (A) 48 : 11-24. 1973 b. The effects of mating on the reproductive biology of three species of Ectobiu~ Stephens (Dictyoptera: Blattidae). Entomologia exp. appl. 16 : 213-222.

CAlUEIZOSr, E. 1957. On the parasites and predators of the cockroach. II. Evania appendigaster (L.). Bull. ont. Res. 48 : 199-209.

CROS, i . 1942. Blatta orientalis et ses parasites, i. Evania punctata Brulld; ii. Eulophus sp. Etude biologique. Eos. Madr. l[S : 45-67.

C~OSSKEY, R . W . 1951. The morphology, taxonomy and biology of the Evanioidea (I-Iymen- optera). Trans. R. ent. Soc. Lend. 102 : 247-301.

DOVTT, R . L . 1959. The biology of parasitic Hymenoptera. A. Rev. Ent. 4 : 161-182. EDMITNDS, L . R . 1952 a. The oviposition of Prosevania punctata (:Brullt). A I tymenopterous

parasite of the cockroach egg capsules. Ohio J. Sci. 52 : 29-30. 1952 b. Some notes on the habits and parasites of native wood-roaches in Ohio (Orthoptera: Blattidae). Ent. News 63 : 141-145. 1954. A study of the biology and life cycle of Prosevanict punctata (:BrulM) with notes on additional species (Hymenoptera: Evaniitlae). Ann. ent. Soc. Am. 47 : 575-592.

ROTE, L. M. & WILl, Is, E . R . 1954. The biology of the cockroach egg parasite, Tetrastichus hagenowii (Hymenoptera: Evaniidae). Trans. Am. ent. Soc. 8{) : 53-72. 1960. The biotic associations of cockroaches. Smithson. misc. Collns. i41 : 1-470.

TOWNES, H. 1949. The nearctic species of Evaniidae (Hymenoptera). Prec. U.S. Nat. Mus. 99 : 525-539.

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